This invention relates to cylindrical locks for doors, and more particularly to a vandal-resistant cylindrical lock apparatus useful in commercial and other public applications.
A cylindrical lock for a door conventionally includes a spring-loaded retractor for retracting and releasing a latchbolt for unlatching and latching the door. The lock body is mounted in a large bore through the door while the latchbolt is housed in an intersecting smaller diameter bore through the edge of the door. The retractor is operated by rotating either one of an inside handle and an outside handle, with the outside handle generally equipped with a key-actuable lock for preventing rotation of the outside handle. Although cylindrical locks are considered to be economical in terms of their manufacture and installation, they are vulnerable to damage by vandals and burglars such as through destructive manipulation of the cylindrical lock's outside handle. The situation is exacerbated for cylindrical locks fitted with lever handles, as may be required for installation on exterior doors in schools and other public buildings in accordance with applicable regulations, since lever handles exert greater torque on the cylindrical lock assembly than do knob handles.
A more secure type of lock apparatus for a door, although more expensive in terms of both manufacture and installation than a cylindrical lock apparatus, is a mortise lock apparatus in which the latching and locking mechanisms are contained in a rectangular case mounted in a rectangular cavity in the edge of the door. A conventional mortise lock assembly is equipped with a lock cylinder, key-actuable from the outside, which enters the lock case independent of the outside handle. When the key is inserted in the mortise lock cylinder and rotated, a correspondingly rotated cam pivots an included tail piece which trips the locking mechanism within the lock case. In some mortise locks, further rotation of the key causes correspondingly further pivoting of the tail piece for tripping the mechanism for unlatching the door. Since only the face of the mortise lock cylinder is exposed outside the door, the lock cylinder is extremely difficult to grab or remove. Further, since the mortise lock mechanism having a locking and unlocking function typically operates independently of the handles, defeating or destroying the outside handle of an installed mortise lock—unlike a conventional cylindrical lock—gets a vandal no closer to gaining unauthorized access.
In view of this background, there has existed a need for a door lock having the economy of manufacture and installation of a cylindrical lock but with the security advantages of a mortise lock.